Three British tourists have been killed after a helicopter crashed in the Grand Canyon over the weekend.

Becky Dobson, Jason Hill, and Stuart Hill died after their helicopter, which was taking them around the canyon, dropped out of the sky around 5.20 p.m. local time (MST) on Saturday, police said.

The aircraft, which was operated by Papillon Grand Canyon Helicopters, fell down towards the western rim of the canyon, Sky News reported.

It's unclear what caused the crash. The US Federal Aviation Administration and National Transportation Safety Board are both investigating the case, the Associated Press reported.

The four survivors — British tourists Ellie Milward, Jonathan Udall, and Jennifer Barham, and pilot Scott Booth — were rescued at 2 a.m. local time on Sunday and airlifted to a hospital in Las Vegas.

Francis Bradley, the chief of the local Hualapai Police, said rescue efforts were hampered by "severe weather conditions," including "gusts up to 50 mph."

Teddy Fujimoto, a Las Vegas photographer in the area at the time, said he saw two or three small explosions in the wreckage, and that one injured woman "was pretty much burned all over."

Sky News

Another witness on Sky News said the helicopter "tilted, fell down, broke in half, crashed, then exploded."

All the tourists were in their late 20s and early 30s.

Brenda Halvorson, the CEO of Papillon Group, said: "It is extreme sadness we extend our heartfelt sympathy ot the families involved in this accident. Our top priority is the care and needs of our passengers and our staff."